1. Field of the Invention
Exemplary aspects of the present invention generally relate to a fixing device, an image forming apparatus including the fixing device, and a fixing method employed in the fixing device, and particularly to a fixing device employing an electromagnetic induction heating method using degaussing coils, and an image forming apparatus including the fixing device.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction devices having two or more of copying, printing, scanning, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of a latent image bearing member (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
One example of the fixing device included in the related-art image forming apparatuses employs an electromagnetic induction heating method in which degaussing coils are provided to prevent an excessive increase in temperature in a non-conveyance portion of a heating member not covered by a sheet.
For example, a fixing device including an induction heating unit provided with an exciting coil and multiple pairs of degaussing coils positioned opposite the exciting coil at both edges of the induction heating unit in a width direction thereof has been proposed. Specifically, the multiple pairs of the degaussing coils are provided at positions corresponding to respective non-conveyance portions of a heating member not covered by each type of sheets having a different size. The multiple pairs of the degaussing coils degauss magnetic fluxes from the exciting coil generated at the position opposite the multiple pairs of the degaussing coils. As a result, an excessive increase in temperature at the non-conveyance portions of the heating member is prevented.
In another approach, Published Unexamined Japanese Patent Application No. 2005-321642 (hereinafter referred to as JP-2005-321642-A) discloses a technique in which a temperature sensor (or a contact-type thermistor) that detects a temperature at an edge of a fixing member in a width direction thereof corresponding to a position of a degaussing coil is provided to drive the degaussing coil based on an increase in the temperature detected by the temperature sensor.
In yet another approach, Published Unexamined Japanese Patent Application No. 2007-226126 (hereinafter referred to as JP-2007-226126-A) discloses a technique in which a temperature sensor that detects a temperature at an edge of a pressing member in a width direction thereof corresponding to a position of each of multiple degaussing coils is provided to control an amount of power supplied to the degaussing coils based on the temperature at a non-conveyance portion of the pressing member detected by the temperature sensor.
However, both the number of types of the above-described temperature sensor for controlling an amount of power supplied to the degaussing coil and positions to install the above-described temperature sensors are considerably limited.
Specifically, in the case of the technique disclosed in JP-2005-321642-A in which the contact-type thermistor is provided to detect a temperature at the edge of the fixing member in a width direction thereof, a mark generated when the contact-type thermistor contacts a surface of the fixing member remains on the surface of the fixing member, and that mark appears also on an image fixed to a sheet. Consequently, an expensive contactless temperature sensor such as a thermopile is required to solve the above-described problem.
Further, because a larger number of components including an exciting coil, the degaussing coil, a separation plate, and so forth are densely packed around the fixing member, it is difficult to spare enough space to provide the temperature sensor to detect the temperature at the edge of the fixing member in the width direction thereof.
By contrast, in the case of the technique disclosed in JP-2007-226126-A, as described above, the temperature sensor that detects a temperature at the edge of the pressing member in the width direction thereof corresponding to the position of each of the degaussing coils is provided to control an amount of power supplied to the degaussing coils based on the temperature at the non-conveyance portion of the pressing member detected by the temperature sensor. Accordingly, the problem of JP-2005-321642-A may be solved.
However, because a difference in a temperature between the fixing member and the pressing member is not always kept constant, it is difficult to control the amount of power supplied to the degaussing coils by indirectly detecting an increase in temperature at the non-conveyance portion of the fixing member based on the result detected by the single temperature sensor provided at the edge of the pressing member in the width direction thereof. In other words, an increase in temperature at the non-conveyance portion of the fixing member may fail to be detected accurately, allowing an excessive increase in temperature in the non-conveyance portion of the fixing member and a decrease in temperature at an edge of a conveyance portion of the fixing member covered by the sheet.